Most Citi bikes go ununsed as far as I can tell.
I personally would've rather seen cleaner, faster, quieter and more reliable subways than more advert-bikes. But it's not so sexy for citibank to donate a tiny fraction of the MTA's budget for some billboards/posters.
Thank goodness we have urban transit planners, people with degrees in this stuff. They are heavily, heavily pushing bicycle transit and bike shares. Not because it's 'sexy', but because it works.
You can plop down a bike share station in a matter of days or weeks (the biggest hassle are the community meetings) which affords enormous flexibility; it takes months to redo a bus route, and decades to plan a subway line. Bike share bikes convert a fair number of people over to bike ownership, too - and the presence or more bike riders on the city's streets makes the streets safer for everyone.